A drive is generally used in vehicles for acceleration and for overcoming air resistance and rolling resistance which occur on acceleration and when driving at constant speed. Due to traffic conditions, therefore, situations arise in which the drive does not contribute to movement of the vehicle, for example when the vehicle is coasting. If an internal combustion engine is used to produce drive, fuel is consumed even when the drive is not producing any motive power. With fuel cell-driven vehicles also, there are situations in which merely the electrical temporary storage device serves as the tractive energy source and, for reasons of operating economy, the fuel cell unit ought to be operated in standby mode.
Conventionally, motor vehicles have been used in which the driver performs all the operations of switching on and off himself/herself, this being done by turning an ignition key or by using an additional button, with which key or button the driver is able to control the operating state of the drive. Automated methods recognize that the engine may be turned off or the fuel cell unit is being put into standby mode from clutch engagement operations in the case of manually shifted transmissions, for example, on the basis of the freewheeling selected. For the fuel cell unit, the standby mode is also referred to as the idle or freewheeling mode. The internal combustion engine or the fuel cell unit is re-started when the clutch is actuated again.
Automatic control of that kind, however, is based on clutch signals which do not occur, for example, in vehicles with automatic transmissions. A problem encountered in the case of automatic control is that, if the internal combustion engine is turned on and off (i.e., started and stopped) infrequently, it runs in situations in which it is possible to dispense with the internal combustion engine, with the result that an unfavorable energy balance is obtained. If, however, the internal combustion engine is switched off and re-started whenever it is under no load, even if only briefly, this results in severe wear to the engine and especially to the starter, which leads to higher maintenance costs and reduced lifetime. This also applies to fuel cell units (in place of the internal combustion engine), which may not be switched on and off (i.e., changed from standby mode to active mode and vice versa) with unlimited frequency without the fuel cell units working inefficiently or, in the long term, being damaged.
Since, in vehicles with automatic transmissions, situations in which the internal combustion engine is to be switched off are not readily distinguishable from situations in which the internal combustion engine should continue to run, it is difficult to optimize the control of the internal combustion engine. In particular, if in the case of automatic transmissions the internal combustion engine is stopped every time the vehicle is stationary and the brake pedal is actuated, this results in a markedly greater strain on the starting system due to the greater number of starting operations. This applies in equal measure to fuel cell-driven vehicles, which by their nature have no gear shifting.
Especially in start/stop systems with automatic transmissions, in hybrid vehicles or in fuel-cell driven vehicles, optimization of the control of the main drive unit is difficult.